1. Field of the Invention
This invention relates generally to coupling between combline and ceramic resonators.
2. Description of Related Art
Cavity resonators are electronic components that produce oscillations at a specified frequency. Cavity resonators can be fashioned so that only certain combinations of electric and magnetic fields exist within the cavity. Such cavities are useful because they can filter out electromagnetic field energy that occurs at undesired frequencies.
A resonant cavity can be structured so that only particular modes of an electromagnetic field are utilized within the cavity. A dielectric post may be placed within the cavity, with its longitudinal axis extending out from a sidewall of the cavity, so as to be substantially perpendicular to the direction of flow of electromagnetic field energy within the cavity. Such posts impose boundary conditions on the electric and magnetic fields, in addition to the behavior imposed by the electrically conducting metallic material of the cavity resonator's walls.
For a ceramic resonator, the term dielectric post is used here to mean a non-metallic puck, a short cylinder of ceramic material, held away from a wall of the cavity by a support. The longitudinal axis of the dielectric puck is substantially perpendicular to the direction of flow of electromagnetic field energy within the cavity resonator. The puck may be shaped as a disk, having a circular cross-section, but could also be designed to have other shapes.
Because the post material is ceramic, the cavity can resonate in a transverse electric (TE) mode, in particular the TE011 mode. In such a mode, in a cavity resonator with a ceramic puck, the electric field will be purely azimuthal with respect to the central axis of the ceramic puck and largest within the ceramic puck. Because the walls of the cavity resonator are metallic, the electric field will decrease in intensity away from the ceramic puck, vanishing at the walls of the cavity. On the other hand, the magnetic field will be orthogonal to the electric field and will have no azimuthal component anywhere in the cavity resonator.
As is evident from the above description of the electric and magnetic fields, if a ceramic cavity is physically adjacent to a metallic cavity, and no special structure is used to couple the two cavities, then the axis of the ceramic puck in the ceramic cavity must be perpendicular to the axis of the metallic cavity. It also must be perpendicular to the direction of flow of energy so that either the magnetic fields or the electric fields in the two cavities align. If this is not done, there can be no flow of energy between the cavities because the magnetic and electric fields in the second cavity can only exist in an orientation not possible for the corresponding fields in the first cavity.
There are several known ways to couple dissimilar cavities, such as metallic combline and ceramic resonators. One approach involves mechanical orientation of physically adjacent cavities, but this technique fixes the layout of the cavities, resulting in complex structures if multiple cavities are used. Another coupling technique uses either a probe-to-probe structure to draw the electric field from one cavity into an orientation suitable for the physically adjacent cavity, or a loop-to-loop structure to perform a similar alignment operation on the magnetic field. A probe-to-loop structure would allow the electric field in one cavity to induce a magnetic field in the physically adjacent cavity. However, these probe and loop structures have the drawback that they may be used only for relatively narrow bandwidth filters because the electric coupling they provide is relatively weak.
U.S. Pat. No. 6,081,175 to Duong et al. discloses a coupling structure for coupling cavity resonators. However, the coupling between dissimilar resonators disclosed by this reference cannot be easily controlled. Accordingly, what is needed is a structure that controllably couples dissimilar resonators, such as ceramic and metallic combline resonators, without fixing the relative orientations of the dissimilar resonators.